Off to the Item World!
The Item World can be accessed from the Overlord's Castle.
As long as it is an item, whether it is a healing item or whatever, it will have a world inside it.
However, be careful when going to the Item World of a high ranked item.
The stronger the item is, the stronger the residents of the Item World will be.
You travel through the Item World on a ship!
There are many types of ships, and the better your ship, the deeper into the items you can go.
You will be able to get more ships as the game progresses, so find a ship you like!